M. J. Highland

In situ characterization of strontium surface segregation in epitaxial La0.7Sr0.3MnO3 thin films as a function of oxygen partial pressure

Using in situ synchrotron measurements of total reflection x-ray fluorescence, we find evidence of strontium surface segregation in (001)-oriented La0.7Sr0.3MnO3 thin films over a wide range of temperatures (25–900 °C) and oxygen partial pressures (pO2=0.15–150 Torr). The strontium surface concentration is observed to increase with decreasing pO2, suggesting that the surface oxygen vacancy concentration plays a significant role in controlling the degree of segregation. Interestingly, the enthalpy of segregation becomes less exothermic with increasing pO2, varying from −9.5 to −2.0 kJ/mol. In contrast, the La0.7Sr0.3MnO3 film thickness and epitaxial strain state have little impact on segregation behavior.

Functional links between stability and reactivity of strontium ruthenate single crystals during oxygen evolution

In developing cost-effective complex oxide materials for the oxygen evolution reaction, it is critical to establish the missing links between structure and function at the atomic level. The fundamental and practical implications of the relationship on any oxide surface are prerequisite to the design of new stable and active materials. Here we report an intimate relationship between the stability and reactivity of oxide catalysts in exploring the reaction on strontium ruthenate single-crystal thin films in alkaline environments. We determine that for strontium ruthenate films with the same conductance, the degree of stability, decreasing in the order (001)>(110)>(111), is inversely proportional to the activity. Both stability and reactivity are governed by the potential-induced transformation of stable Ru(4+) to unstable Ru(n>4+). This ordered(Ru(4+))-to-disordered(Ru(n>4+)) transition and the development of active sites for the reaction are determined by a synergy between electronic and morphological effects.

In situ synchrotron x-ray characterization of ZnO atomic layer deposition.

The utility of in situ synchrotron x-ray scattering and fluorescence in gaining insight into the early stages of the atomic layer deposition process is demonstrated in this study of ZnO growth on Si. ZnO films are found to initially grow as islands, with the onset of coalescence occurring during the fourth growth cycle. The start of coalescence is accompanied by a small increase in surface roughness. After ten cycles of growth, the growth rate decreases from 4.2 to 3.0 A per cycle, with the growth following expected self-limiting behavior. The overall growth process is consistent with the model of Puurunen and Vandervorts for substrate-inhibited growth [R. L. Puurunen and W. Vandervorst, J. Appl. Phys. 96, 7686 (2004)].

In situ synchrotron x-ray studies of strain and composition evolution during metal-organic chemical vapor deposition of InGaN.

Composition and strain inhomogeneities strongly affect the optoelectronic properties of InGaN but their origin has been unclear. Here we report real-time x-ray reciprocal space mapping that reveals the development of strain and composition distributions during metal-organic chemical vapor deposition of InxGa1−xN on GaN. Strong, correlated inhomogeneities of the strain state and In fraction x arise during growth in a manner consistent with models for instabilities driven by strain relaxation.

Full-field X-ray reflection microscopy of epitaxial thin-films

Novel X-ray imaging of structural domains in a ferroelectric epitaxial thin film using diffraction contrast is presented. The full-field hard X-ray microscope uses the surface scattering signal, in a reflectivity or diffraction experiment, to spatially resolve the local structure with 70 nm lateral spatial resolution and sub-nanometer height sensitivity. Sub-second X-ray exposures can be used to acquire a 14 µm × 14 µm image with an effective pixel size of 20 nm on the sample. The optical configuration and various engineering considerations that are necessary to achieve optimal imaging resolution and contrast in this type of microscopy are discussed.

Ultrafast terahertz-field-driven ionic response in ferroelectric BaTiO3

The dynamical processes associated with electric field manipulation of the polarization in a ferroelectric remain largely unknown but fundamentally determine the speed and functionality of ferroelectric materials and devices. Here we apply subpicosecond duration, single-cycle terahertz pulses as an ultrafast electric field bias to prototypical BaTiO3 ferroelectric thin films with the atomic-scale response probed by femtosecond x-ray-scattering techniques. We show that electric fields applied perpendicular to the ferroelectric polarization drive large-amplitude displacements of the titanium atoms along the ferroelectric polarization axis, comparable to that of the built-in displacements associated with the intrinsic polarization and incoherent across unit cells. This effect is associated with a dynamic rotation of the ferroelectric polarization switching on and then off on picosecond time scales. These transient polarization modulations are followed by long-lived vibrational heating effects driven by resonant excitation of the ferroelectric soft mode, as reflected in changes in the c-axis tetragonality. The ultrafast structural characterization described here enables a direct comparison with first-principles-based molecular-dynamics simulations, with good agreement obtained. (Less)

X-ray nanodiffraction of tilted domains in a poled epitaxial BiFeO3 thin film

We present measurements of crystallographic domain tilts in a (001) BiFeO3 thin film using focused beam x-ray nanodiffraction. Films were ferroelectrically pre-poled with an electric field orthogonal and parallel to as-grown tilt domain stripes. The tilt domains, associated with higher energy (010) vertical twin walls, displayed different nanostructural responses based on the poling orientation. Specifically, an electric field applied perpendicular to the as-grown domain stripe allowed the domain tilts and associated vertical twin walls to persist. The result demonstrates that thin film ferroelectric devices can be designed to maintain unexpected domain morphologies in working poled environments.

Phase stabilization of δ-Bi2O3 nanostructures by epitaxial growth onto single crystal SrTiO3 or DyScO3 substrates

We observe that the high-temperature δ-phase of Bi2O3 is stabilized to room temperature by the epitaxial growth of nanostructures onto either (001)-oriented SrTiO3 or (001)p-oriented DyScO3 single crystal substrates. In addition, the morphology can be controlled by the miscut of the substrate. Synchrotron x-ray scattering observations at controlled temperatures and oxygen partial pressures reveal that the δ-Bi2O3 nanostructures are coherently strained to the substrates at room temperature. Annealing the nanostructures at 600 °C causes gradual conversion of the (001)-oriented δ-phase to an unidentified strain-relaxed phase.

Mesoscopic structural phase progression in photo-excited VO2 revealed by time-resolved x-ray diffraction microscopy

Dynamical phase separation during a solid-solid phase transition poses a challenge for understanding the fundamental processes in correlated materials. Critical information underlying a phase transition, such as localized phase competition, is difficult to reveal by measurements that are spatially averaged over many phase separated regions. The ability to simultaneously track the spatial and temporal evolution of such systems is essential to understanding mesoscopic processes during a phase transition. Using state-of-the-art time-resolved hard x-ray diffraction microscopy, we directly visualize the structural phase progression in a VO2 film upon photoexcitation. Following a homogenous in-plane optical excitation, the phase transformation is initiated at discrete sites and completed by the growth of one lattice structure into the other, instead of a simultaneous isotropic lattice symmetry change. The time-dependent x-ray diffraction spatial maps show that the in-plane phase progression in laser-superheated VO2 is via a displacive lattice transformation as a result of relaxation from an excited monoclinic phase into a rutile phase. The speed of the phase front progression is quantitatively measured, and is faster than the process driven by in-plane thermal diffusion but slower than the sound speed in VO2. The direct visualization of localized structural changes in the time domain opens a new avenue to study mesoscopic processes in driven systems.

Interfacial charge and strain effects on the ferroelectric behavior of epitaxial (001) PbTiO3 films on (110) DyScO3 substrates

In-situ synchrotron x-ray observations reveal that the ferroelectric behavior of epitaxial (001) PbTiO3 thin films grown on (110) DyScO3 substrates depends on both film thickness and interfacial electrical properties. A 92-nm-thick film was found to exhibit an a/c domain structure with a ferroelectric Curie temperature similar to that theoretically predicted based on the strain state. In contrast, 6-nm-thick films contained only c-oriented domains, and the ferroelectric behavior was found to depend strongly on the nature of the electrical boundary condition at the buried interface.